and does it matter
The link to kirkegaard's rebuttal is broken.
“attributing the burning down of the White House during the War of 1812 to the Canadians rather than the British”
I actually heard that from a Canadian. He then laughed and said “as part of the British empire, of course”. Troops did come, some of them, from Canada.
I share your concern with the issue of determining truth in a sea of falsehoods.
Perhaps unlike you, I'm unsurprised when people say things that are not true, even high status individuals speaking on the record. I've never encountered anyone who does otherwise; any differences are of degree and/or predominant motivation.
One source of false statements you don't mention is popular mythology. "Everyone knows" that Paul Revere warned people that "the British are coming"; they probably learned it in elementary school. Except, historians say, the colonists would have considered themselves British; it's more likely he warned of "the regulars", "the army", or similar. We know the Puritans sought religious freedom, and generally don't know that this included the freedom to enforce their particular sect on others. We know about Pocahantas. Only pedants and professional historians insist on relating the actual truth rather than the popular story.
There are similar myths outside of history. Some are known across whole cultures; others are simply known to and believed by most everyone in particular fields. These myths appear without attribution in works that are otherwise decently footnoted. Politicians repeat them, whether or not they actually believe them.
The problem is that when someone repeats myths like these in areas where you know better, that tells you little or nothing about whether they are ignorant or merely prone to using popular soundbites. They may be careful with facts in areas they have reason to consider uncertain, while sloppy with "common knowledge". Or on the other hand, they may not be.
There's also the popular habit of saying "people/we/category do/think x" and meaning "lots of people/us/category do x" or even "members of (category) are more likely to do/think x than average". Lying? Sloppy thinking? A clever debate technique, as the meaning gradually shifts from "some people" to "all people"? Or perhaps honest use of what the speaker sees as common shorthand?
Meanwhile, people who don't do both of these at least some of the time appear vanishingly rare to me. So I can't presume that everything they say is unusually unreliable, unless I've caught them in exceptional levels of untruth, particularly motivated untruth.
I'd like to hold politicians and other high status individuals to higher standards than some guy I meet in the bar, but while they may on average, be better educated than average (status correlates with education), they don't seem to do any better about producing truth.
And that's before you get those who define Truth in interesting ways, such as "whatever the highest status person on my side claims" or "whatever feels right to my gut". (And even *those* people often attend to ordinary truth some of the time, rather than the things they otherwise claims as Truth.)
It frustrates me how much people just believe politicians and journalists. In the end we are all human and biased. Some of us are dishonest not matter our profession. I am always glad when some examples are shown but it’s sadly not often enough !
By saying "every politician is a liar," you are supporting the biggest liars. And the ones who want to undermine democracy.
But maybe your point is just to say everyone is stupid except you.